Use Microsoft Security Essentials to guard against viruses, spyware, and other evil software. It provides real-time protection for your home or small business PCs. Microsoft Security Essentials is free* and we designed it to be simple to install and easy to use. It runs quietly and efficiently in the background so you don’t have to worry about interruptions or making updates.
This free Security tools from Microsoft provide to user with legal Microsoft Windows, before download Microsoft Security Essential, Microsoft validate your computer license. But there are another solution for user who not yet activated their Microsoft Windows by downloading from this link :
x64 : http://mse.dlservice.microsoft.com/download/A/3/8/A38FFBF2-1122-48B4-AF60-E44F6DC28BD8/enus/amd64/mseinstall.exe
x86 : http://mse.dlservice.microsoft.com/download/A/3/8/A38FFBF2-1122-48B4-AF60-E44F6DC28BD8/enus/x86/mseinstall.exe
x64 : http://download.microsoft.com/download/DefinitionUpdates/mpam-fex64.exe
x86 : http://download.microsoft.com/download/DefinitionUpdates/mpam-fe.exe